Currently, 3GPP has been conducting studies on Long Term Evolution (LTE)-Advanced, which is an advanced version of LTE. In LTE-Advanced, a heterogeneous network (HetNet) has been examined as a technique to satisfy system performance requirements such as spectral efficiency (system capacity) and energy saving of base stations (each may be called an “eNB”). In HetNet, in addition to the cell arrangement around a macro cell, which is a traditional high transmission-power base station, as the center of the network, a cell which is a low transmission-power base station such as a picocell or femtocell (small cell) is disposed in a macro cell (see, e.g., Non-Patent Literature (hereinafter, referred to as “NPL” 1). Small cells are each used as a radio interface that efficiently provides coverage for a local area such as an indoor facility where a large amount of traffic is generated in a concentrated manner, for example.
In arrangement of a large number of small cells for supporting the traffic expected to continue increasing, techniques to control switching between a state where a small cell temporarily stops transmission processing (OFF state) and a state where a small cell performs transmission processing (ON state) have been examined for the purpose of reducing power consumption of a base station on the network side or alleviating interference from a small cell.
For example, when the traffic of a base station (macro-cell) is high, transmission from a small cell starts (turn on transmission) to off-load the traffic to the small cell. Meanwhile, when the traffic of a base station is low, transmission from a small cell is stopped (turn off transmission). The operation to achieve power saving and reduction of interference to a neighboring cell by a small cell is expected. For example, FIG. 1 illustrates a configuration of a communication system in which cells 2 to 4 (which are small cells) are disposed in the coverage area of cell 1 (which is a macro-cell). In FIG. 1, cells 1 and 2 are cells performing transmission (hereinafter, referred to as an “ON-state cell”) while cells 3 and 4 are cells stopping transmission (hereinafter, referred to as an “OFF-state cell”).
The network side that controls switching between the ON-state and OFF-state of a small cell determines which one of OFF-state cells is to be configured to an ON-state, based on a reception-quality measurement-result (may be called a “measurement report”) reported from a terminal (may be called a “UE”) (i.e., using the report from a terminal as a trigger for switching between the ON and OFF states). When transmission of all signals from the OFF-state cells is stopped, the terminal can no longer detect the OFF-state cells. For this reason, in order to allow the terminal to detect the OFF-state cells, the OFF-state cells transmit a signal for the terminal to detect the base stations (cell-detection-signal (may be called a “discovery signal”)). Upon detection of a cell (base station) using the cell-detection-signal, the terminal reports, to the network side, a reception-quality measurement-result acquirable using the measured reception quality for the cell.
3GPP has been studying use of a new cell-detection-signal (may be called a “new cell-detection-signal” hereinafter), which has a higher detection accuracy than the traditional cell-detection-signal (may be called an “old cell-detection-signal” hereinafter), as a cell-detection-signal transmitted from an OFF-state cell.
This new cell-detection-signal is expected to have no backward compatibility. For this reason, a traditional terminal (e.g., terminal supporting LTE Release 11 or before Release 11) cannot detect a base station using the new cell-detection-signal. As a result, since the traditional terminal cannot detect an OFF-state cell, the terminal cannot measure the reception quality of the OFF-state cell to trigger switching the OFF-state cell to the ON state on the network side. Meanwhile, a new terminal (e.g., terminal supporting LTE Release 12) can detect a base station using the new cell-detection-signal transmitted from an OFF-state cell and measure the reception quality of the base station. For this reason, the new terminal can trigger switching the OFF-state cell to the ON-state on the network side.
In addition, 3GPP has been studying a technique for a terminal to add an offset value to a reception-quality measurement-value in order for the terminal to execute a handover to a specific cell soon (e.g., NPL 2). When a reporting condition for reporting a reception-quality measurement-result to a cell (serving cell) to which the terminal is connected is satisfied, the terminal reports the reception-quality measurement-result. In this case, use of the reception-quality measurement-result acquirable by adding an offset value to the reception-quality measurement-value of the cell measured by the terminal makes it possible to satisfy the reporting condition soon as compared with the case where the reception-quality measurement-value is used alone. Thus, the serving cell can trigger a handover soon based on the reception-quality measurement-result reported from the terminal. This offset value is configured for each cell and the offset values are indicated to each terminal using terminal-specific signaling.